Programmable current sources are some of the most versatile components used in analog technology. They can be used in a variety of applications including analog computation, offset cancellation, parameter adjustment measurements, characterization of devices, driving actuators, and in Automatic Test Equipment (ATE).
In ATE applications, precise programmable current sources are necessary for precision parametric measurement units and integrated circuit quiescent current (IDDQ) measurements. The operating parameters in these applications necessitate precise current control, because the ATE system may be used as the reference for testing integrated circuits (ICs). Specifically, it has been known that manufacturing defects in the semiconductor fabrication process can be detected by precise measurement of current.
One of the most common implementations of a current source couples an operational amplifier, also referred to as an “op-amp”, with a transistor and a resistor. The polarity of the output current distinguishes current sinks, current sources, and combined current sink/sources. A current sink draws current like a load and can only have current flowing in via its output pin. A current source can only have current flowing out of its output pin. A current sink/source may have current flowing into or flowing out of its output pin, that is, current may be measured as a negative or positive value.
FIG. 1 is a diagram of an exemplary prior art programmable current sink 100. In FIG. 1, a reference voltage supply (REF) 101 is coupled with a digital-to-analog converter (DAC) 102. The output of DAC 102 is coupled with the non-inverting input 110 of an op-amp 103. The output of op-amp 103 is coupled with a resistor 105 through the gate of transistor 104. In FIG. 1, the inverting input 111 of op-amp 103 is coupled with the source of transistor 104. Op-amp 103 regulates the gate of transistor 104 so that the voltage drop across resistor 105 is essentially the same as the voltage output by DAC 102. In other words, there is a 0 volts difference in potential between non-inverting input 110 and inverting input 111. The reference voltage supplied by reference voltage supply 101 is regulated by DAC 102 according to the digital bit value to which it is set. Thus, a set voltage (VSET) is output from DAC 102 referenced to ground and which is sed to regulate the amount of current flowing into current sink 100 via output pin 120. The current flowing through resistor 105 can be derived by the equation:I=Vprog/R where R is the resistance value of resistor 105, Vprog is the program voltage supplied by DAC 102 as seen across resistor 105. The minimum output voltage for current sink 100 can be expressed by the equation:Vout(min)=Vprog+VDS(sat). VDS(sat) is the saturation voltage of transistor 104. If a high impedance load, connected to the output of current sink 100, generates a voltage below Vout(min) the current source will become unregulated. Vout(min) is directly proportional to the programmed current and has an upper limit of:Vout(min)=Vref+VDS(sat). Vref is the maximum output voltage of DAC 102 which is bounded by its REF_LO, in this Figure tied to ground, and its REF_HI, in this Figure supplied by reference voltage supply 101.
Current sinks of the types just described have had several problems and limitations associated with their use. For example, one drawback of system 100 is the limitation on output voltage as described above. One method for preventing the DAC from putting out voltages above a certain limit (e.g. Vref/2), is by limiting the use of the programming bits available to the DAC. However, this results in a reduction in resolution for this type of current sink.
A second possibility would be to reduce the reference Voltage Vref. Since errors due to noise, offset, and drift essentially stay the same, they may become significant in comparison to the desired output voltage. Thus the accuracy of the voltage output by DAC 102 is then determined by the error signals rather than least significant bit used to program the DAC. Thus the ability of the prior art as shown in current sink 100 to precisely control current is limited in applications requiring low output voltage.
FIG. 2 shows an exemplary prior art implementation of an automatic test equipment system 200. A digital signal processor (DSP) 202 is coupled with an analog to digital converter (ADC) 201 and with a plurality of digital to analog converters 102. DSP 202 reads data from ADC 201 and sends digital signals to the DACs which are used to control the output from the DACs. Typically, automatic test systems are used to perform parametric testing of integrated circuits. This necessitates precise control of current and voltage in order to obtain accurate test results and to prevent damage to the circuits being tested.
As mentioned above, the program voltage can be lowered by limiting the number of programming bits used by DAC 102. For example, DSP 202 can send digital signals to DAC 102 that only cause DAC 102 to utilize 4 of its programming levels. While this can effectively limit the voltage output from DAC 102, it also reduces the dynamic range of the DAC and limits the ability to precisely control current in some applications.
The exemplary prior art of FIG. 1 can also be reconfigured as shown in FIG. 3 to create a current source. In FIG. 3, a reference voltage supply (REF) 304 is coupled with a digital-to-analog converter (DAC) 303. The output of DAC 303 is coupled with the non-inverting input of an op-amp 302. The output of op-amp 302 is coupled with a resistor 305 through the gate of transistor 306. In FIG. 3, the inverting input of op-amp 302 is coupled with the source of transistor 306.
The reference voltage supplied by reference voltage supply 304 is regulated by DAC 303 according the digital bit value to which it is set. The output current is driven by the reference voltage supplied by reference voltage supply 304. The feedback to the inverting input of op-amp 302 adjusts the gate voltage so that the sensed voltage matches the output of the DAC.
VDS (sat) is the saturation voltage of transistor 306. Vref is the maximum output voltage of DAC 303 which is bounded by its REF_HI. One drawback to the current source design of FIG. 3 is that the current range desired by entering the highest values of binary code to the DAC may be unreachable. For example, the maximum value of Vref output by the DAC may not be applied across the resistor 305 because there is necessarily a voltage across the transistor 306. This translates into a negative output voltage which might not be tolerable by the load. Thus, the maximum Iout current represented by setting the DAC to its full limit is not attainable.
FIG. 4 is a diagram of an exemplary current sink/source. Current sink/source 400 exhibits the same limitations as current sink 100 of FIG. 1 with respect to low output voltage (e.g., susceptibility to error and loss of resolution). In addition, another problem of the prior art is that to provide both current sink and current source capability, DAC 403 must provide both positive voltage when acting as a current source and a negative voltage when acting as a current sink or vice versa. Each programming bit of the DAC 403 now controls twice as much voltage, thus further aggravating the loss of resolution due to the unavailability of the highest order bits and reducing the precision with which current can be controlled. Alternatively, to realize the same level of precision as the current sink of FIG. 1, 2 DACs or a 2 output DAC (e.g., DAC 403 of FIG. 4) are needed, thus increasing the cost of the circuit. However, the use of the programming bits available to the DAC is still limited which results in a reduction in resolution for this type of current sink/source.
FIG. 5 is a diagram of an exemplary prior art precision current sink/source 500 that can overcome the problem of constrained voltage swing exhibited in current sink/source 400. In FIG. 5, differential amplifier 501 is used in conjunction with feedback amplifier 502 to control current. The output voltage generated by the load external to the system attached to pin 540 is sensed by feedback amplifier 502 and fed back into the reference input of differential amplifier 501. As the output voltage changes due to varying load impedance, differential amplifier 501 adjusts the voltage supplied to resistor 504. The formula for the voltage across resistor 504 can be expressed as:Vprog=Vset−Vout, Where Vprog is the voltage drop across resistor 504 and Vout is the output voltage at output pin 540. As Vout changes, the feedback causes Vset to closely track these changes, thus maintaining the same Vprog across the resistor.
However, the part count in precision current sink/source 500 is higher due to the additional resistors and op-amp in differential amplifier 501. Thus, the overall precision of current sink/source 500 is affected by these additional parts. The higher part count also makes current sink/source 500 more expensive and more complex for manufacturers to fabricate.